The following information is provided to assist the reader to understand the technology described below and certain environments in which such technology can be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the technology or the background thereof. The disclosures of all references cited herein are incorporated by reference in their entirety.
A typical pH sensor based on potentiometric principles includes a reference electrolyte solution, an indicating electrode immersed in or in contact with an analyte solution (of which the pH is to be measured), a reference electrode immersed in the reference electrolyte solution, and measurement circuitry such as potentiometric circuitry in electrical connection with the reference electrode and the indicating electrode. The potentiometric circuitry measures the electrical difference between the indicating and reference electrodes. Ionic contact between the electrolyte solutions in which the indicating electrode and the reference electrodes are immersed provides electrical connection between the electrodes. The pH value of the sample or analyte electrolyte solution (which is proportional to concentration of the hydrogen ions in the sample electrolyte) is directly correlated with the potential difference developed at the indicating electrode following the Nernst equation.
In the above-described configuration, an important condition for correct measurement is that the electric potential difference built up in the reference electrode and the reference electrolyte is maintained constant such that the reading from the potentiometric circuitry solely represents the potential difference in the indicating electrode, that is, pH in the electrolyte solution. To meet this condition, a common arrangement is to have the reference electrode immersed in a saturated reference electrolyte solution, and to have a small “window” positioned between the saturated reference electrolyte solution and the sample or analyte electrolyte solution to provide ionic contact and thus an electrical connection between the saturated reference electrolyte solution and the sample or analyte electrolyte solution. The “window” is usually fabricated from a porous material such as a porous glass membrane, a hydrophilic porous polymer membrane, etc. Because of the porosity of the “window”, a non-negligible mass exchange occurs between the saturated reference electrolyte solution and the sample or analyte electrolyte solution, thereby causing cross-contamination in both solutions.
The dilution of the saturated reference electrolyte solution resulting from such contamination can be a significant problem since it changes the potential difference in the reference electrode. The contamination also deteriorates the stability of the pH sensor and shortens the lifetime of the pH sensor. As the dimensions of a pH sensor are reduced (for example, to very small, microlevel, microscale or smaller dimension), the problem is exacerbated because the volume of the saturated reference electrolyte solution is very small compared to the sample electrolyte solution. For example, for applications where a microscale or smaller pH sensor is implanted into a human body and is utilized to measure a physiological pH (for example, myocardial pH), the volume of the saturated reference electrolyte solution is extremely small compared to the volume of the myocardial tissue of which the pH is to be measured. At such a scale, the saturated reference electrolyte solution is diluted much more quickly than in a macro scale glass tube type pH sensor.
Another factor which affects the useful life of a pH sensor, such as a microscale pH sensor, is the durability of the reference electrode. In many instances, conductive material of the reference electrode is gradually dissolved and consumed into the saturated reference electrolyte solution. At some point during the dissolution and consumption of the reference electrode, the useful life of the pH sensor is terminated.